The present invention relates to a process for the aqueous suspension polymerization of vinyl chloride monomer (VCM), this being one of the most widely known and used methods for producing vinyl chloride polymers (PVC).
The method usually involves mixing liquid VCM with water and with an initiator of free radical polymerization, while the monomer itself is kept in dispersion by stirring at a high temperature, until polymerization has been obtained to the desired degree of conversion.
As widely known, it is most important to obtain a PVC having:
besides high porosity of the granules, PA0 a rather uniform granulometric distribution, that is to say, a minimum quantity of too small or too large particles, or none at all.
The importance of obtaining high porosity of the granules derives not only from technical requirements--such as that for the granules to absorb plasticizers rapidly and in great quantity for forming "dry blends"--but also from health reasons, in that it has been noticed that only with highly porous granules is it possible to obtain a more thorough removal of the VCM residues, which have proved to be carcinogenic.
The importance of obtaining a rather uniform granulometric distribution derives essentially from technical requirements, particularly those of the subsequent working stages, for the production of finished or semifinished products.
The combination of the two aforespecified characteristics, though having been notoriously desirable for a long time, is not easy to obtain, even if the specialized literature describes various processes for carrying out polymerization in such a way as to achieve this object.
This problem, as yet unsolved on an industrial scale, is particularly acute in the case of polymerization in reactors equipped with turbine stirrers.
As described in U.S. Pat. No. 3,772,226, the properties of the PVC obtained are influenced by different variables, depending on the technique used for suspension polymerization. As evidenced in this patent, the porosity of the polymer granules tends to clearly diminish if polymerization is carried on up to more than 90% of monomer conversion; hence, the known processes of suspension polimerization are interrupted at about 85% of the monomer conversion.
As described in the same U.S. Pat. No. 3,772,226, it is known that the use of suspending agents, in suspension polymerization, may notably influence the characteristics of the polymer obtained: whereby a great variety of substances, generally hydrosoluble, have been suggested for this purpose.
It has also been proposed to use a combination of "primary" and "secondary" or "auxiliary" suspending agents, in order to obtain an acceptable compromise between porosity and size of the particles. It is known that the "primary" suspending agents act as regulators of the particle size: they are usually hydrosoluble substances, such as cellulose or gelatin derivatives, polyvinyl alcohols, maleic anhydride/vinyl acetate copolymers, and others. The "secondary" suspending agents, which have been used with the intention of increasing the porosity of the granules, comprise partial esters of fatty acids with polyalcohols, monoglycerides of unsaturated fatty acids, polyethylene glycol esters, polysaccharides, and others.
In order to reduce the negative effect--on the granulometric uniformity of the particles--of the secondary suspending agents, it has been proposed to add other ingredients, as for example aliphatic alcohols and acids, acting as surfactants apt to control the monomer spreading coefficient on water (see for example Italian Pat. No. 674.764), or an exactly proportioned mixture of primary suspending agents, emulsifying agents (chiefly anionic) and inorganic coagulants (as described in the Italian Pat. No. 562.180).
A substantial improvement on the known technique has been obtained by means of the process according to our pending application U.S. Ser. No. 115,376 filed Jan. 28, 1980 now abandoned, in which it has been proposed to use the combination of a conventional suspending agent with a secondary suspending agent consisting of partially hydrolyzed polyvinyl acetate, having a saponification number between 300 and 500. This allows one to obtain the twofold advantage of being able to carry out polymerization with monomer conversion up to 93% and, at the same time, to keep the porosity of the granules at excellent values, and hence with very little VCM residues in the final product.
Since this important result was our chief object--in connection with the aforementioned health problems--less importance was given to the fact that the use of the specified secondary suspending agent led to the production of a polymer with rather wide granulometric distribution and, in particular, with an undesired increase in the percentage of the finer particles.